In the field of telecommunications, the synthesis of speech is of particular interest. It permits people unskilled in computer technology to receive so-called canned messages, e.g. by telephone, without the necessity of employing full-time human operators or of using costly subscriber terminals. Such messages may inform a calling subscriber of congestion at an exchange, of the cost and duration of a call, and of a changed directory number.
In commonly owned U.S. Pat. No. 4,319,084, in the names of Paolo Lucchini et al, there has been disclosed a multichannel digital speech synthesizer designed to simulate the human voice as closely as possible. The patent also mentions several prior publications relating to speech analysis.
As further disclosed in that prior patent, a programmed message source such as a computer stores sets of processing parameters which can be transmitted in a predetermined sequence to a signal generator for commanding the emission of excitation pulses of varying amplitudes and polarities to a lattice filter; these parameters represent coded information relating to frequency distribution, volume and duration of speech elements such as diphones, i.e. phoneme pairs. Several identical input modules are inserted in parallel between the computer, on the one hand, and the signal generator and the filter, on the other hand, and are associated with respective output channels which carry speech samples produced by the filter after they have been converted from digital to analog form. A validity-interval counter and a sound-interval counter in each input module are presettable by the computer through the intermediary of a pair of alternately written and read buffer memories. The sound-interval counter measures the length of an operating period of a periodic signal, representing voiced sound, or an aperiodic (pseudorandom) signal, representing unvoiced sound, available from respective read-only memories in the signal generator; the validity-interval counter controls the switchover between the reading and writing phases of the two buffer memories. A further memory in each input module temporarily stores weighting coefficients and sound-intensity data obtained under the control of the sound-interval counter from whichever buffer memory is enabled for reading. All the elements of the synthesizer are controlled by a common time base. The operation of the synthesizer is based upon the presumption that the various filter coefficients, representing the effects of reflection between different cavities of an acoustic tube designed as a model of the human vocal tract, can be considered constant during short time intervals on the order of 10 ms. These filter coefficients are updated only at the beginning of an oscillatory cycle of a voiced-sound interval whose pitch is determined by the setting of the sound-interval counter.